Evaporation vessel apparatus and method

ABSTRACT

Disclosed is a method of providing a constant concentration of a metal-containing precursor compound in the vapor phase in a carrier gas. Such method is particularly useful in supplying a constant concentration of a gaseous metal-containing compound to a plurality of vapor deposition reactors.

METHOD AND APPARATUS

This Application is a Continuation of application Ser. No. 13/959,699,filed on Aug. 5, 2013, which is a Divisional of application Ser. No.12/749,048, filed on Mar. 29, 2010, now U.S. Pat. No. 8,501,266, whichclaims the benefit of priority of Provisional Application Ser. No.61/211,274, filed on Mar. 27, 2009.

The present invention relates generally to a method, and apparatus for,delivering precursors in the vapor phase to epitaxial reactors. Inparticular, the present invention relates to a method, and apparatus,for maintaining concentration of a precursor in the vapor phase relativeto a carrier gas.

Chemical vapor deposition (“CVD”) processes are used in the electronicsindustry, particularly the semiconductor industry, to deposit layers ofmaterial. Precursors, which may be solid or liquid, are typicallyprovided in cylinders. In use, a carrier gas enters the cylinder, passesthrough the precursor, becomes saturated with the precursor, and thenthe carrier gas/precursor vapor mixture exits the cylinder and isdirected to a deposition reaction chamber. In the deposition chamber, alayer or film of the precursor is grown on a substrate.

Typically, cylinders (also referred to as bubblers or more generally asevaporation vessels) are made of stainless steel and have a gas inletextending into and below the surface of the precursor. This gas inlet iscommonly referred to as a “dip-tube”. High purity carrier gas passesthrough the dip tube, bubbles up through the liquid precursor, andtransports the precursor vapor to the deposition reactor.

For most CVD processes it is essential to dispense exactly meteredamounts of precursor vapor (usually measured in mmol/min or some otherconvenient unit) to the reaction chamber. The common method to deliverprecise amounts of precursor vapor relies on tight control of thetemperature of the precursor and the total pressure in the evaporationvessel (cylinder). FIG. 1 illustrates a conventional high-performanceprecursor vaporization apparatus for CVD processes having a carrier gasfeed line 1, a mass flow controller 2 in the carrier gas feed line,precursor cylinder 3 contained within a temperature regulating chamber4, precursor cylinder 3 containing precursor 6, and having a dip-tube 5for directing the carrier gas into precursor cylinder 3 below the levelof precursor 6, gas exit line 7 for directing carrier gas/precursorvapor mixture away from precursor cylinder 3 and to reaction chamber 9,gas exit line 7 containing optional concentration transducer 8 whichtransmits a signal to electronic controller 10 which in turn adjustsmass flow controller 2 (according to the equation {dot over (m)}={dotover (m)}₀+A(c−c₀) with calibration constant A). The mass flowcontroller 2 is composed of a mass flow transducer and a gas flow valve.The precursor temperature is controlled by maintaining the cylinder in atemperature regulated chamber. Evaporating liquid precursor in thecylinder will slightly lower the temperature of the liquid and willaccordingly lower the concentration of the precursor in the vapor phaserelative to the carrier gas. For most CVD systems which use small-sizedcylinders to supply precursor compound to a single reaction chamber,such slight temperature drop does not appreciably affect theconcentration of the precursor in the vapor phase. Any change inconcentration can be adjusted for by increasing the mass flow of thecarrier gas into the bubbler but for the majority of installations thechanges are so miniscule that concentration transducer 8 is omitted.This approach provides a constancy of precursor concentration in thevapor phase that is better than ±0.5% of a set value (e.g., for aprecursor concentration set value of 10 mol %, the vapor phaseconcentration ranges from 9.95% to 10.05%), which is acceptable for thelarge majority of CVD processes.

There is a trend in the industry to move to larger-sized cylinders,which reduces equipment down time because cylinders are changed lessoften. Such larger-sized cylinders are also increasingly used to supplya precursor to one or more deposition reactors. Also, specialized CVDprocesses require a more active control of the vapor concentration tocompensate for uneven evaporation of precursor (due to evaporativecooling of the precursor liquid in the cylinder). When a cylindersupplies precursor to multiple reactors, compensating for reducedvaporized precursor concentration relative to the carrier gas byincreasing the flow of the gas mixture does not solve the problem ofreduced precursor concentration. For example, each reactor might berunning a different deposition process and may have differentconcentration requirements. Also, the information for proper adjustmentof the mass flow may not be available unless a concentration transduceris used.

Apparatuses for delivering vaporized precursor to a plurality ofdeposition reactors using a single precursor source cylinder are known.For example, International Patent Application WO 2001/42539 (Ravetz etal.) discloses a method and apparatus for delivering vaporized precursorto a plurality of epitaxial reactors which utilizes conventional massflow controllers to adjust the flow rate to each epitaxial reactor. Thisapproach of Ravetz is conventional in that it relies on adjustment ofthe mass flow and does not have any means to compensate for vapor phaseprecursor concentration changes. This approach fails to providevaporized precursor concentration control required by advanced vapordeposition methodologies

The present invention, which addresses the deficiencies of conventionalprocesses, provides a method of depositing a film on a substrate,including: (a) providing an evaporation vessel including a chambercontaining a precursor compound to be vaporized, the evaporation vesselhaving a gas inlet and a gas outlet, a carrier gas feed line in fluidcommunication with the gas inlet, a gas control valve, and a gas exitline in fluid communication between the gas outlet and one or more vapordeposition reactors, the gas exit line having a pressure transducer anda concentration transducer, each of the gas control valve, pressuretransducer, and concentration transducer in electrical connection with acontroller; (b) conveying a gaseous mixture including vaporizedprecursor compound and a carrier gas to the one or more vapor depositionreactors; (c) maintaining a substantially constant concentration of theprecursor compound in the gaseous mixture by the steps selected from thegroup consisting of: (1) sensing a concentration of the vaporizedprecursor compound in the gaseous mixture in the gas exit line;comparing the sensed concentration (c) with a reference concentration(c₀) to provide a concentration differential (c−c₀); generating a signalin the controller utilizing the concentration differential; transmittingthe signal to the gas control valve wherein the signal adjusts the gascontrol valve to adjust the total pressure within the evaporation vesselin order to maintain a substantially constant concentration of thevaporized precursor compound in the gaseous mixture in the gas exitline; (2) providing the evaporation vessel with a temperature sensingmeans, the temperature sensing means situated so as to sense temperatureof precursor compound; sensing the temperature of the precursorcompound; comparing the sensed temperature (T) with a referencetemperature (T₀) to provide a temperature differential (T−T₀);generating a signal in the controller utilizing the temperaturedifferential; transmitting the signal to the gas control valve whereinthe signal adjusts the gas control valve to adjust the total pressurewithin the evaporation vessel in order to maintain a substantiallyconstant concentration of the vaporized precursor compound in thegaseous mixture in the gas exit line; and (3) a combination of (1) and(2); and (d) subjecting the gaseous mixture to conditions in the one ormore deposition reactors sufficient to deposit a film.

The present invention further provides a system for delivering asubstantially constant concentration of a vaporized precursor compoundin a carrier gas including an evaporation vessel including a chambercontaining a precursor compound to be vaporized, the evaporation vesselhaving a gas inlet and a gas outlet, a carrier gas feed line in fluidcommunication with the gas inlet, and a gas exit line in fluidcommunication between the gas outlet and one or more vapor depositionreactors; a gas control means; a sensing means in the gas exit line forsensing a concentration of the vaporized precursor compound in thegaseous mixture in the gas exit line; means for comparing the sensedconcentration (c) with a reference concentration (c₀) to provide aconcentration differential (c−c₀); signal generating means forgenerating a signal in a concentration controller utilizing theconcentration differential; and a means for transmitting the signal tothe gas control valve wherein the signal adjusts the gas control valveto adjust total pressure in the evaporation vessel in order to maintaina substantially constant concentration of the vaporized precursorcompound in the gaseous mixture in the gas exit line.

The present invention also provides a system for delivering asubstantially constant concentration of a vaporized precursor compoundin a carrier gas including an evaporation vessel including a chambercontaining a precursor compound to be vaporized, the evaporation vesselhaving a gas inlet and a gas outlet, a carrier gas feed line in fluidcommunication with the gas inlet, a gas exit line in fluid communicationbetween the gas outlet and one or more vapor deposition reactors, and atemperature sensing means situated within the evaporation vessel so asto sense temperature of precursor compound; means for comparing thesensed temperature (T) with a reference temperature (T₀) to provide atemperature differential (T−T₀); a gas control means; means forgenerating a signal in a controller utilizing the temperaturedifferential, and a means for transmitting the signal to the gas controlvalve wherein the signal adjusts the gas control valve to adjust totalpressure in the evaporation vessel in order to maintain a substantiallyconstant concentration of the vaporized precursor compound in thegaseous mixture in the gas exit line.

FIG. 1 illustrates a conventional precursor vaporization apparatus forCVD processes.

FIG. 2 illustrates an apparatus of the invention having a concentrationsensing means.

FIG. 3 illustrates an apparatus of the invention having a temperaturesensing means.

FIG. 4 illustrates an apparatus of the invention suitable for solidprecursor compounds.

In the figures, like reference numerals refer to like elements.

The present invention provides a method of providing a gaseous mixtureof a precursor compound and carrier gas to one or more vapor depositionchambers (or reactors), and preferably to a plurality of reactors,wherein the gaseous mixture has a substantially constant concentrationof vaporized precursor compound. By “substantially constantconcentration” is meant a vapor phase concentration of ±1% of areference concentration, preferably ±0.5% of the referenceconcentration, more preferably ±0.3%, and yet more preferably ±0.25%(for example, for a precursor concentration set value of 10 mol %, thevapor phase concentration preferably ranges from 9.975% to 10.025% mol%). Any suitable carrier gas, which may be reactive or non-reactive, maybe used in the present invention. The particular choice of carrier gasdepends upon a variety of factors, including the precursor compound usedand the particular chemical vapor deposition system employed. Examplesof carrier gases include, without limitation, hydrogen, helium,nitrogen, argon and mixtures thereof. Hydrogen and nitrogen arepreferred.

As used herein, the term “precursor compound” refers to any compoundused to provide a film-forming element in the vapor phase to a vapordeposition reactor, particularly a reactor for chemical vapordeposition. Exemplary film-forming elements include, without limitation,metals, metalloids, and carbon. Precursor compounds useful in thepresent invention may be either liquids or solids under the vaporizationconditions employed. For example, solid precursor compounds having a lowmelting point may be kept in the liquid state by heating the cylinder.Preferably, the precursor compound is a liquid under the evaporationconditions. Suitable precursor compounds include metalorganic compounds.As used herein, the term “metalorganic compounds” also includesmetalloid-organic compounds, that is organic compounds containing ametalloid element such as silicon, germanium, phosphorus, bismuth, andantimony. Exemplary precursor compounds include, without limitation,trimethylgallium, triethylgallium, trimethylaluminum, trimethylindium,dimethylzinc, silane, dichlorosilane, boron trichloride, isobutylgermane, and germanium tetrachloride.

In typical operation, the precursor compound is placed in an evaporationvessel and the evaporation vessel is then placed in a vapor deliveryapparatus. Evaporation vessels may be constructed of any suitablematerial, such as glass, polytetrafluoroethylene or metal, as long asthe material is inert to the precursor compound contained therein.Metals are preferred, particularly nickel alloys and stainless steels.Suitable stainless steels include, but are not limited to, 304, 304 L,316, 316 L, 321, 347 and 430. Suitable nickel alloys include, but arenot limited to, INCONEL, MONEL, and HASTELLOY. It will be appreciated bythose skilled in the art that a mixture of materials may be used in themanufacture of such evaporation vessels.

A carrier gas enters the evaporation vessel through a gas inlet opening,which may be at the top or the bottom of the evaporation vessel. In thecase of a liquid precursor, the carrier gas typically passes through adip-tube extending into and below the surface of the precursor. As thecarrier gas exits the dip-tube, it bubbles up through the precursorcompound and becomes saturated with precursor compound vapor. Thecarrier gas/precursor compound vapor mixture exits the evaporationvessel through a gas outlet and is conveyed to a deposition reactor.Exemplary evaporation vessels having a dip-tube include those disclosedin U.S. Pat. Nos. 4,506,815 and 5,755,885.

In the case of a solid precursor, the evaporation vessel may contain oneor more chambers and a porous element. The solid precursor is typicallydisposed on the porous element, the porous element typically being thefloor or a part of the floor of a chamber containing the precursorcompound. The carrier gas may pass up through the porous element andthen through the solid precursor compound. Alternatively, the carriergas may pass first through the precursor compound and then through theporous plate. As the carrier gas passes through the precursor compoundit picks-up vaporized precursor to form a gas stream including vaporizedprecursor admixed with carrier gas. The amount of vaporized precursorpicked-up by the carrier gas may be controlled. It is preferred that thecarrier gas is saturated with vaporized precursor. The carrier gas exitsthe chamber containing the precursor compound and optionally enters anoutlet chamber which is in fluid contact with the inlet chamber beforeexiting the evaporation vessel through an outlet opening. Exemplaryevaporation vessels for solid precursor compounds include thosedisclosed in U.S. Pat. Nos. 4,704,988 (Mellet); 5,603,169 (Kim); and6,607,785 (Timmons et al.).

The carrier gas may be used at a wide variety of flow rates. Such flowrates are a function of the evaporation vessel cross-sectionaldimension, pressure and system demands. Larger cross-sectionaldimensions allow higher carrier gas flows, i.e. linear velocity, at agiven pressure. The carrier gas flow entering the evaporation vessel,exiting the evaporation vessel or both entering and exiting the vesselmay be regulated by a control means. Any suitable control means may beused, such as manually activated control valves or computer activatedcontrol valves.

In use, the evaporation vessel may be used at a variety of temperatures.The exact temperature will depend upon the particular precursor compoundused and desired application. The temperature controls the vaporpressure of the precursor compound, which controls the flux of thematerial needed for specific growth rates or alloy compositions. Suchtemperature selection is well within the ability of one skilled in theart. For example, when the precursor compound is trimethyl indium, thetemperature of the evaporation vessel may be from 10° to 60° C. Othersuitable temperature ranges include from 35° to 55°, and from 35° to 50°C. The evaporation vessel may be heated by a variety of heating means,such as by placing the vessel in a thermostatic bath, by directimmersion of the vessel in a heated oil bath or by the use of ahalocarbon oil flowing through a metal tube, such as a copper tube,surrounding the vessel.

After exiting the evaporation vessel, the precursor compoundvapor/carrier gas mixture is conveyed to a deposition chamber (reactor).The deposition chamber is typically a heated vessel within which isdisposed at least one, and possibly many, substrates. The depositionchamber has an outlet, which is typically connected to a vacuum pump inorder to draw by-products out of the chamber and to provide a reducedpressure where that is appropriate. Chemical vapor deposition can beconducted at atmospheric or reduced pressure. The deposition chamber ismaintained at a temperature sufficiently high to induce decomposition ofthe precursor compound. The deposition chamber temperature is typicallyfrom 200° to 1200° C., the exact temperature selected being optimized toprovide efficient deposition. Optionally, the temperature in thedeposition chamber as a whole can be reduced if the substrate ismaintained at an elevated temperature, or if other energy such as radiofrequency (“RF”) energy is generated by an RF source. Suitablesubstrates for deposition, in the case of electronic device manufacture,include, but are not limited to, silicon, germanium, gallium arsenide,indium phosphide, and sapphire. Such substrates are particularly usefulin the manufacture of various electronic and photovoltaic devices.

As used herein, the term “transducer” refers to the sensor that convertsa physical quantity into an electrical signal. Preferred concentrationtransducers are acoustic concentration sensors that directly measure theconcentration of a binary gas mixture, that is vaporized precursorcompound in a carrier gas. “Controller” refers to the circuitry orsoftware that utilizes the input of a transducer in combination with areference value to adjust an actuator (or valve) in a prescribed way. Atransducer and a controller may be provided as individual units or as anintegrated unit. Integrated transducer and controller units aregenerally commercially available, such as the EPISON™ 4 concentrationmonitor available from Aixtron AG (Aachen, Germany) or the PIEZOCON™controller from Lorex Industries, Inc. (Poughkeepsie, N.Y.).

From a cost and infrastructure standpoint, it is desirable to use onecentral vapor delivery system for multiple reactors and oneconcentration transducer and one controller for the entire installation.FIG. 2 illustrates such an apparatus for delivering a precursor compoundvapor-carrier gas mixture having evaporation vessel 15 containing liquidprecursor compound 16, such as trimethylgallium, and having carrier gasinlet 17, gas outlet 18, dip-tube 19 and gas exit tube 20, evaporationvessel 15 contained within temperature regulating chamber 21. Carriergas is provided to the evaporation vessel through feed line 22, havinggas control means (valve) 23, which is connected to carrier gas inlet 17and is in electrical connection with controller 29. In use, the carriergas passes through control valve 23, enters evaporation vessel 15through carrier gas inlet 17 and exits dip-tube 19 and bubbles upthrough precursor compound 16 to form a gas stream of a mixture ofprecursor compound vapor and carrier gas. This gas stream then exits theevaporation vessel through exit tube 20, passes through gas outlet 18and through gas exit line 24 toward a plurality of reaction chambersillustrated as 25 a, 25 b, and 25 c, although less than 3 or more than 3reaction chambers may be present. Gas exit line 24 has pressuretransducer 26, concentration transducer 28 and pressure release valve32, each of these in electrical connection with controller 29. Whenexcess pressure builds in the system, a signal is transmitted fromcontroller 29 to pressure release valve 32 which then adjusts the totalpressure by releasing (or venting) a portion of the gas stream.

Dip-tube 19 is shown extending upwards from the bottom of evaporationvessel 15, extending above the surface of precursor compound 16, forminga u-bend and then extending downward below the surface of precursorcompound 16. It will be appreciated by those skilled in the art thatvarious dip-tube configurations are possible, such as extending downwardfrom the top of the evaporation vessel toward the bottom of the vesselor extending inward from a side of the evaporation vessel and bendingdownward toward the bottom of the vessel. In operation, the gas exit endof the dip-tube must be below the surface of the precursor compound.

Concentration transducer 28 is typically an acoustic concentrationsensor which senses the concentration of the precursor compound vapor inthe gas stream, generates a signal and sends this signal to controller29. The controller then compares the sensed concentration (c) with areference concentration (c₀) to provide a concentration differential(c−c₀), generates a signal utilizing this concentration differential,and transmits the signal to gas control valve 23 wherein the signaladjusts gas control valve 23 to adjust the total pressure withinevaporation vessel 15 in order to maintain a substantially constantconcentration of the vaporized precursor compound in the gaseous mixturein gas exit line 24. The precursor compound vapor referenceconcentration is programmed into controller 29. Such referenceconcentration will vary depending upon the particular precursor compoundused, the type and number of deposition reactor employed, and theparticular film being deposited in the deposition reactor. Theparticular reference concentration input is well within the ability ofone skilled in the art.

When concentration transducer 28 senses a change in concentration, thecontroller 29 adjusts the total pressure accordingly by acting on gascontrol means (valve) 23 and restores the vaporized precursor compoundconcentration in the gas stream to the reference concentration value. Asuitable equation for adjusting the pressure using the “integral”capability of a commercial PID (proportional integral differential)controller is:

$p = {p_{o} + {B{\int\limits_{time}\left( {c - c_{o}} \right)}}}$where in P₀ is the reference pressure, c₀ is the reference concentrationand B is a calibration constant. Each of p₀, and c₀ are programmed intocontroller 29. The reference concentration c₀ is the desiredconcentration of vaporized precursor compound in the gas stream. Inorder to maintain a substantially constant vaporized precursor compoundconcentration in the gas stream, sensed concentration c is maintainedsuch that it is substantially equal to the reference concentration c₀.

Unlike the conventional approach of acting on the carrier gas mass flowinto the evaporation vessel, controller 29 acts on the total pressure ofthe system in order to correct for precursor compound concentrationfluctuations in the gas exit line. This method leads to small pressurefluctuations in the central delivery system. These small pressurefluctuations introduced by the control system will not have an adverseeffect on the performance of the mass flow controllers in the reactors.While the total pressure in the gas exit line is adjusted, it isimportant that the total pressure remain sufficiently high for a massflow controller inside the CVD system to work properly.

Concentration transducers can be quite expensive so an alternateapparatus for maintaining a substantially constant concentration ofprecursor compound vapor in the gas stream is also contemplated by thepresent invention. FIG. 3 illustrates an alternate apparatus fordelivering a precursor compound vapor-carrier gas mixture havingevaporation vessel 15 containing liquid precursor compound 16, such astrimethylaluminum, and having carrier gas inlet 17, gas outlet 18,dip-tube 19, gas exit tube 20 and temperature sensor 31, evaporationvessel 15 contained within temperature regulating chamber 21. Carriergas is provided to the evaporation vessel through feed line 22, havinggas control valve 23, which is connected to carrier gas inlet 17 and inelectrical connection with controller 29. In use, the carrier gas passesthrough control valve 23, enters evaporation vessel 15 through carriergas inlet 17 and exits dip-tube 19 and bubbles up through precursorcompound 16 to form a gas stream of a mixture of precursor compoundvapor and carrier gas. This gas stream then exits the evaporation vesselthrough exit tube 20, passes through gas outlet 18 and through gas exitline 24 toward a plurality of reaction chambers illustrated as 25 a, 25b, and 25 c, although less than 3 or more than 3 reaction chambers maybe present. Gas exit line 24 has pressure transducer 26, concentrationtransducer 28 and pressure release valve 32, each of these in electricalconnection with controller 29.

Temperature sensing means (or sensor) 31 is located within evaporationvessel 15 so as to sense temperature of the precursor compound.Temperature sensing means 31 may be any suitable sensor, such as athermocouple. The temperature sensing means may be constructed out ofany suitable material that is non-reactive with the precursor compound.

Concentration transducer 28 is typically an acoustic concentrationsensor which senses the concentration of the precursor compound vapor inthe gas stream, generates a signal and sends this signal to controller29. Temperature sensing means 31 senses a temperature of the precursorcompound in evaporation vessel 15, generates a signal and sends thissignal to controller 29. The controller then compares the sensedtemperature (T) with a reference temperature (T₀) to provide atemperature differential (T−T₀), generates a signal utilizing thistemperature differential, and transmits a signal to gas control valve 23wherein the signal adjusts gas control valve 23 to adjust the totalpressure within evaporation vessel 15 in order to maintain asubstantially constant concentration of the vaporized precursor compoundin the gaseous mixture in gas exit line 24.

In this embodiment, the total pressure is adjusted using the actual,measured temperature of the evaporating liquid precursor compound. Thechange of the vapor pressure with temperature is well known. Using thetemperature measurement, the total pressure is changed by the samefraction as the vapor pressure and as a result the concentration willremain constant. Reference temperature T₀ and reference pressure P₀ areinputted into controller 29. These inputs are used to determine thedesired precursor compound concentration in the gas stream. If theprecursor compound evaporation rate changes (either detected viaconcentration transducer 28 or temperature sensor 31), the totalpressure is adjusted in order to compensate. The change of the totalpressure is relatively fast, such as requiring only a few seconds. Usinga reference temperature as an input, a suitable control equation for thepressure controller using the “proportional” capability of acommercially available PID controller is p=p₀+D(T−T₀) where p₀ is thereference pressure, p is the pressure, D is a calibration constant, T isthe sensed temperature and T₀ is a reference temperature. Normally, thetemperature dependence of the vapor pressure is linearized for thereference temperature. With the use of a digital controller thetemperature dependence of the vapor pressure can be programmed and usedfor any reference temperature and no recalibration is required.

FIG. 4 illustrates an apparatus for delivering a precursor compoundvapor-carrier gas mixture having evaporation vessel 34 having carriergas inlet 35 and gas outlet 36, evaporation vessel 34 having inletchamber 38 having a floor containing porous element 39 in fluidcommunication with outlet chamber 37, and solid precursor compound 41,such as trimethylindium, contained within inlet chamber 38. Evaporationvessel 34 is contained within temperature regulating chamber 42. FIG. 4shows evaporation vessel 34 containing optional temperature sensingmeans 40, which is in electrical connection with controller 29. Any ofthe temperature sensing means discussed above are suitable for use witha solid precursor compound.

Porous element 39 is typically a frit having a controlled porosity.Porous elements having a wide variety of porosities may be used. Theparticular porosity will depend upon the a variety of factors wellwithin the ability of one skilled in the art. Typically, the porouselement has a pore size of from about 1 to about 100 microns, preferablyfrom about 1 to about 10 microns. However, porous elements havingporosities greater than 100 microns may be suitable for certainapplications. Any material may be used to construct the frit provided itis inert to the organometallic compound used and the desired porositycan be controlled. Suitable materials include, but are not limited to,glass, polytetrafluoroethylene or metals such as stainless steels ornickel alloys. It is preferred that the porous element is sinteredmetal, and more preferably stainless steel. The suitable stainlesssteels and nickel alloys suitable for preparing the porous element arethose described above for the manufacture of the cylinder.

Carrier gas is provided to the evaporation vessel through feed line 22,having gas control means (valve) 23, which is connected to carrier gasinlet 35 and is in electrical connection with controller 29. In use, thecarrier gas passes through control valve 23, enters evaporation vessel34 through carrier gas inlet 35 and enters inlet chamber 38. The carriergas then percolates through solid precursor compound 41 and entrains orpicks-up precursor compound vapor forming a gas stream of a mixture ofvaporized precursor compound and carrier gas. The gas stream then passesthrough porous element 39, enters outlet chamber 37 and exits theevaporation vessel through outlet 36. Next, the gas stream is conveyedthrough gas exit line 24 toward a plurality of reaction chambersillustrated as 25 a, 25 b, and 25 c, although less than 3 or more than 3reaction chambers may be present. Gas exit line 24 has pressuretransducer 26, concentration transducer 28 and pressure release valve32, each of these in electrical connection with controller 29. Whenexcess pressure builds in the system, a signal is transmitted fromcontroller 29 to pressure release valve 32 which then adjusts the totalpressure by releasing (or venting) a portion of the gas stream.

Controller 23 may be either an analog or programmable digitalproportional integral differential controller. Digital controllers arepreferred given their economy, flexibility and availability.

Any suitable gas control means may be used in the present invention.Typically, the gas control means 23 is a control valve. In FIGS. 2 and3, gas control means 23 is shown in gas feed line 22. Alternatively, thegas control means may be present in gas exit line 24. As a furtheralternative, two gas control means may be used, one in the gas feed lineand one in the gas exit line. Preferably, the gas control means is inthe gas feed line. When two gas control means are used, one such meansis static (that is, set to a particular value) and the other gas controlmeans is used to adjust the total pressure in order to maintain asubstantially constant concentration of vaporized precursor compound inthe gas mixture, and preferably the control means in the gas exit lineis static.

Alternatively, a substantially constant concentration of precursorcompound concentration in the gas stream can be maintained by acombination of the above methods, namely sensing precursor compoundtemperature and comparing the sensed temperature with a referencetemperature to provide a temperature differential, and sensing precursorcompound concentration and comparing the sensed concentration with areference concentration to provide a concentration differential,generating a signal in a controller using the temperature differentialand the concentration differential, and transmitting the signal to thegas control valve wherein the signal adjusts the gas control valve toadjust total pressure within the evaporation vessel in order to maintaina substantially constant concentration of the precursor compound vaporin the gaseous mixture in the gas exit line.

It will be appreciated by those skilled in the art that the apparatus ineither FIG. 2 or FIG. 3 may optionally contain a precursor compoundfill-port. Such a fill-port allows for periodic addition of additionalprecursor compound, such as from a separate precursor compoundreservoir. In this way, the evaporation vessel may remain in continuoususe with less system downtime which would be the result when an emptyevaporation vessel is replaced with a full one. Such fill-ports aretypically used in evaporation vessels used to deliver liquid precursorcompound in the vapor phase.

One advantage of the method and apparatus described herein is that asubstantially constant concentration of vaporized precursor compound inthe gas stream is maintained rather than maintaining a constant massflow of carrier gas into the evaporation vessel. Control of theprecursor compound concentration is more direct and eliminates the needto change the mass flow. There is no need in the present apparatus for amass flow transducer in the gas feed line, and preferably there is nosuch mass flow transducer in the gas feed line. A further advantage ofthe present invention is that precursor compound vapor generation andvaporized precursor compound concentration control can be containedwithin the vapor delivery apparatus. This is extremely desirable ascontrol is centralized in the vapor delivery apparatus rather than beingspread out over the central delivery system and several the several CVDreactors.

What is claimed is:
 1. An apparatus for delivering a substantially constant concentration of a vaporized precursor compound in a carrier gas to a plurality of vapor deposition reactors, comprising: an evaporation vessel comprising a chamber containing a precursor compound to be vaporized, the evaporation vessel having a gas inlet and a gas outlet, a carrier gas feed line in fluid communication with the gas inlet, a gas exit line in fluid communication between the gas outlet and the plurality of vapor deposition reactors, and a temperature sensing means situated within the evaporation vessel so as to sense temperature of the vaporized precursor compound; means for comparing the sensed temperature (T) with a reference temperature (To) to provide a temperature differential (T-To) of the vaporized precursor compound; wherein the means for comparing comprises a controller; a gas control means in the carrier gas feed line; wherein the gas control means comprises a gas control valve and the gas control valve is electrically connected to the controller; means for generating a signal in the controller utilizing the temperature differential; a means for transmitting the signal to the gas control means wherein the signal adjusts the gas control means to adjust a total pressure in the evaporation vessel in order to maintain the substantially constant concentration of the vaporized precursor compound in a gaseous mixture in the gas exit line; and with no mass flow transducer in the carrier gas feed line, wherein the gas exit line further comprises a pressure transducer and a concentration transducer; wherein the concentration transducer in the gas exit line senses a concentration of the vaporized precursor compound in the gaseous mixture in the gas exit line; wherein the pressure transducer and the concentration transducer are electrically connected to the controller and said controller is configured to maintain the substantially constant concentration of the vaporized precursor compound in the gaseous mixture in the gas exit line with steps selected from the group consisting of : 1) sensing a concentration of the vaporized precursor compound in the gaseous mixture with the concentration transducer in the gas exit line, comparing the sensed concentration with a reference concentration to provide a concentration differential, generating a signal in the controller utilizing the concentration differential, transmitting the signal to the gas control valve wherein the signal is configured to adjust the gas control valve to adjust the total pressure within the evaporation vessel in order to maintain a substantially constant concentration of the vaporized precursor compound in the gaseous mixture in the gas exit line; 2) sensing the temperature of the vaporized precursor compound within the evaporation vessel with the sensing means, comparing the sensed temperature with a reference temperature to provide a temperature differential, generating a signal in the controller utilizing the temperature differential, transmitting the signal to the gas control valve wherein the signal adjusts the gas control valve to adjust the total pressure within the evaporation vessel in order to maintain a substantially constant concentration of the vaporized precursor compound in the gaseous mixture in the gas exit line; and (3) a combination of (1) and (2).
 2. The apparatus of claim 1 wherein a second gas control valve is present in the gas exit line.
 3. The apparatus of claim 1 wherein the gas exit line further comprises a pressure release valve.
 4. The apparatus of claim 1, wherein the temperature sensing means comprises a temperature sensor, wherein the means for generating a signal comprises the controller, and wherein the means for transmitting the signal comprises the controller. 